1. Field of the Invention
The present invention relates to an automatic transmission assembly for a power unit of a vehicle. More particularly, the present invention relates to an automatic transmission assembly configured and arranged for shortening a gear shifting time in both upshifting and downshifting of gears, and for controlling gear shifting for a partially neutral position on a shift drum of the automatic transmission assembly.
2. Description of the Background Art
There are a number of known automatic transmissions assemblies having a twin clutch assembly including two clutches (a first clutch and a second clutch) disposed between a crankshaft of an engine and the transmission unit. In these known automatic transmission assemblies, concurrently with a gear shifting operation performed by an actuator, the first clutch and the second clutch are alternately engaged and disengaged to successively change gears, without interrupting the transmission of drive power from the engine.
An example of such a known automatic transmission assembly is disclosed in Japanese Laid-Open Patent No. 2006-52748, which discloses a transmission unit capable of shifting between two adjacent gear positions by changing engaged and disengaged states of first and second clutches when a shift drum for actuating shift forks is in a certain angular position.
While the drive power from the crankshaft of the engine is being transmitted by the transmission in a certain gear position, a preliminary gear shifting action is performed for angularly moving the shift drum to an angular position, in advance, depending on a next gear position in an upshifting direction for shortening a gear upshifting time.
However, the transmission assembly disclosed in Japanese Laid-Open Patent document No. 2006-52748 is aimed at shortening the gear shifting time for upshifting only, and does not address shortening a gear shifting time for downshifting. Also, a detailed timing for actuating a twin clutch assembly for shortening the gear shifting time for upshifting is not considered in the Japanese Patent document No. 2006-52748.
The present invention has been made to overcome such drawbacks. Accordingly, it is one object of the present invention to provide an automatic transmission assembly which overcomes drawbacks of the transmission assembly as disclosed in the Japanese Patent document No. 2006-52748, and which is capable of shortening a gear shifting time for both upshifting and downshifting of gears.
Further, there are known transmissions for shifting gears with a shift fork which can be actuated in response to angular movement of a shift drum. Such transmissions include a sensor for detecting the angular position of the shift drum and a present gear position.
An example of such known transmissions for shifting gears with a shift fork is discussed in the Japanese Patent No. 3325843, which discloses an arrangement in which a sensor having a fixed contact point and a movable contact point is disposed on an axial end of a shift drum, and a present gear position is detected based on a signal from the sensor.
However, the Japanese Patent No. 3325843 does not address a transmission having a partially neutral position intermediate between given angular positions corresponding to respective gear positions for temporarily reducing the angular speed of a shift drum upon gear shifting, and does not consider detecting a partially neutral position and performing corresponding gear shifting control.
Accordingly, it is also an object of the present invention to provide an automatic transmission assembly which solves the above problems of the Japanese Patent No. 3325843, and which is capable of performing gear shifting control corresponding to a partly neutral position of a shift drum.